1. Field of the Invention
The present invention relates in general to a clutch for a vehicle such as an automobile and more particularly to a clutch pedal depression assisting mechanism for assisting or boosting depression on a clutch pedal.
2. Description of the Prior Art
Two different types of mechanisms for assisting depression of a clutch pedal are the diaphram spring type and the coil spring type. In general, a diaphram spring type clutch unit has such a clutch pedal depressing force characteristic as indicated by a dotted line curve "T" in FIG. 5. In order to reduce the necessary clutch pedal depressing force and thereby make it possible to operate the clutch with less effort, a so-called turnover mechanism, servomotor, etc. is used. In many small-sized cars, the turnover mechanism is used.
An example of a turnover mechanism is disclosed in Japanese Provisional Patent Publication No. 55-55022 and constructed so that an assist coil spring resists rotation of a clutch pedal upon rotation of same between a rest position and a turnover or transition position and assists the rotation upon rotation of the clutch pedal exceeding the turnover position. This mechanism effects such a depressing force variation charactreristic as indicated by a one-dot chain line curve "R0" in the graph of FIG. 6A. In the graph, indicated by "A" is the turnover position.
In the above described turnover mechanism, an increase in the force exerted by the assist spring services to increase the depression assisting for. This however increases the necessary pedal depressing force "V" (refer to FIG. 5) at the begining of rotation of the clutch pedal. In order to solve this problem, it has been proposed to employ a second assist spring as disclosed in Japanese Provisional Utility Model Publication No. 61-39635. The second assist spring exerts such a pedal depression assiting force as indicated by a two-dot chain line in FIG. 6B. The two assist springs effect such a combined assisting force variation characteristic as indicated by the two-dot chain line "R1" in FIG. 6A. The dotted line curve "T" in FIG. 5 indicates an example of a depressing force variation characteristic or a clutch operating force variation charactristic of a clutch unit of itself. The two-dot chain line curve R3 is obtained by combining the characteristic curves "R1" and "T".
A disadvantage of the turnover mechanism having two assist springs is that the return speed of the clutch pedal becomes slower by the provision of the second spring. This will be apparent from the comparision between the chracteristic curves "R0" and "R1" since the turnover position "A" is moved into position "A'" which corresponds to a rotated position of the clutch pedal nearer to its rest position. Another disadvantage is that a feel of operation of the clutch pedal is not good since the necessary pedal depressing force varies largely in response to variation of the pedal stroke as will be understood from the characteristic curve "R3".